This invention relates to iron-base alloy compositions and methods for preparing these compositions. In a preferred embodiment, the compositions and methods relate to nickel containing austenitic ferrous alloy compositions, especially low nickel compositions. In a more preferred aspect, this invention relates to dopants added to low nickel austenitic alloys as a means of improving the elevated temperature oxidation resistance. This invention may be extended to apply to cast alloys.
Low nickel ferrous alloys containing chromium and having nickel present in amounts of about 5 to 15 percent by weight and having manganese, nitrogen and carbon present to aid in forming and stabilizing any austenite present are known.
Compared to traditional cast iron construction, sheet metal automotive exhaust system parts would offer the advantages of both lighter weight and reduced thermal mass. To maximize these advantages, the metal thickness of wrought automotive engine parts such as thermal reactors and turbocharger housings, should be minimized. This can be accomplished by constructing the engine parts from stainless steels, austenitic where hot strength is required, and with alloying suitable for resistance to deterioration by engine exhaust gases on the inside surface of the engine parts and atmospheric air on the outside surface of the engine parts where the surface operating temperature is at a maximum. Such a construction is not cost effective because the resistance to oxidation of the lower cost stainless steel sheet metal alloys at elevated temperatures of 1,500 degrees F to 2,200 degrees F is not sufficient to allow their use in applications where the alloy is exposed to the combustion products normally formed by gasoline fueled internal combustion engines. Because the presently available low-cost alloys do not resist oxidation in elevated temperature combustion environments, it is necessary to use a more expensive alloy with a higher nickel and/or chromium content in automotive emission control devices such as thermal reactors. Therefore, the limitation to using currently available, adequate alloy content stainless steels is the high cost and excessive strategic element content.
Degradation of alloy materials, such as stainless steels, at elevated temperatures is largely dependent on the protective capacity of surface oxide films formed from the alloy during exposure to heat in oxygen containing atmospheres. In one respect, this invention deals with an effective method of improving the protective capacity of oxide scales formed on alloy materials such as low nickel austenitic stainless steels.
By way of summary, the compositions of the present invention relate to the discovery that certain elements can be added to iron-base alloy materials to dramatically improve their resistance to oxidation. More particularly, the invention relates to the discovery that the addition of these elements (referred to herein as "dopants") yields lower cost materials suitable for use in heretofore impractical environments. The compositions of the present invention comprise iron-base alloy compositions exhibiting improved resistance to oxidation comprising:
(i)iron;
(ii) at least one alloy element selected from the group consisting of nickel, chromium, molybdenum, manganese, silicon, carbon, vanadium, cobalt, copper, nitrogen, aluminum, titanium, zirconium, and mixtures thereof; and
(iii) an effective amount of a dopant selected from the group consisting of lithium, sodium, potassium, yttrium, lanthanum, cerium, calcium, magnesium, barium, aluminum, beryllium, strontium, and mixtures thereof.
The oxidation problems of the currently available alloy materials can be overcome by using alloy materials, such as low nickel austenitic (LNA) stainless steel alloys containing chromium and ferritic stainless steel alloys containing medium to high levels of chromium with the additions of an effective amount, preferably at least about 0.02, and more preferably about 0.1 to 2 percent by weight, of the dopants or doping elements or alloys disclosed herein. Alloy compositions of the present invention would be made in a conventional manner, i.e., typical of the alloy content without the dopant of the present invention, but with provision for the addition of dopant elements, in the melt process or later, in later alloy processing, or by surface treatment.
In the preferred alloys of the present invention, according to this invention, barium, calcium, lithium, lanthanum/cerium, magnesium, potassium and sodium or mixtures thereof are added to the alloy as dopants.
The methods disclosed herein involve the addition of small quantities of elements (appearing for the most part in Groups IA, IIA, and IIB of the Periodic Table of Elements) to the base alloy composition. These elements, as ions, enter into the protective oxide scale and modify predominantly anion and to a lesser extent cation transport through the oxide scale, greatly reducing the amount of oxidation observed due to elevated temperature exposure.
Research leading to this invention was based upon low nickel austenitic (LNA) compositions and was guided by extensive use of experimental design. Initially, a 28 run balanced orthogonal array fractional factorial scheme according to Plackett and Burman was employed as a screening method to identify main-effect influences on oxidation resistance of 26 constituents from the Periodic Table of Elements. For this work, reference was made to an article entitled "The Design Of Optimum Multifactorial Experiments" by R. L. Plackett and J. P. Burman (Biometrika, 1946, pages 305-327) which is hereby expressly incorporated by reference; an article entitled "Some Generalizations In The Multifactorial Design" by R. L. Plackett (Biometrika, 1946, pages 328-332) which is hereby expressly incorporated by reference; and to an article entitled "Table Of Percentage Points Of The T-Distribution" by Elizabeth M. Baldwin (Biometrika, 1946, page 362) which is also hereby expressly incorporated by reference. Selection criteria for elements to be considered included commercial availability in quantities sufficient to support volume alloy production, cost and subject reasoning as to the elements' ability to be a stable part of the alloy composition. Fitting these 26 constituents into the 28 run experimental design left 2 columns for random variation or error determination. The Table I lists these constituents by Periodic Table groupings. Note that La-Ce is considered as one constituent because these two elements co-exist as a commercial rare-earth product. Table II lists elements considered as part of the base composition and therefore not included in the oxidation improvement design scheme.
TABLE I ______________________________________ Periodic Table Group Constituent ______________________________________ IA Li, Na, K IIA Be, Mg, Ca, Sr, Ba IIIA B, Al IVA Si, Sn VA Pb, Sb, Bi IB Cu IIB Zn IIIB Y, La--Ce IVB Ti, Zr VB V, Nb, Ta VIB Mo, W ______________________________________
TABLE II ______________________________________ Periodic Table Group Constituent ______________________________________ IVA C VA N VIB Cr VIIB Mn VIII Fe, Co, Ni ______________________________________
Elements associated with improvements in oxidation resistance, as determined by the Plackett-Burman experimental design, were then incorporated in full factorial experimental designs of the form 2.sup.3 to 2.sup.6 for identification of interactions and 3.sup.2 to 3.sup.3 for quantifying certain effects. The notation Y.sup.X refers to X factors evaluated at Y levels each for a total of Y.sup.X test runs. Similar notations and documentation of full factorial experimental designs and analysis can be found in the literature. For this work, reference was made to "Industrial Statistics" by W. Volk (Chemical Engineering, March 1956) which is hereby expressly incorporated by reference. Interactions in this context are those situations wherein the main effect between certain variables change as a function of changes in other variables.
In the course of this research, it was found that elements within the alloy functioned in three identifiable ways: Austenite Stabilizers, Oxide Formers and Oxide Dopants. Understanding of these functions is helpful in describing the alloys of this invention.
Austenite Stabilizers. Alloys of this invention are designed to maintain a stable austenitic matrix at use temperatures up to 2200 degrees F. Elements identified as promoting this austenite stability are Mn, Co, Ni, Cu, C, Sn, Sb, Bi and N. Throughout the course of this alloy development, it was necessary to periodically adjust the choice and quantity of austenite stabilizer elements to balance the counteracting effects of La-Ce, Ti, Zr, V, Cr, Al and Si as these elements were introduced or changed in concentration as part of the effort to determine their effect on oxidation resistance.
Oxide Formers. An object of this invention is to improve the protective nature of surface oxides formed during exposure to elevated temperatures and, therefore, a stable surface oxide is required. Elements identified as significant contributors to stable surface oxide formation on these iron base alloys are: Cr, Co, Ni, Al and Si. The elements Cr, Co and Ni were, individually or in combination, part of the base composition, Table II, and therefore not subject to evaluation during the fractional factorial phase of this investigation. Al and Si were incorporated into the initial experimental design (Table I) and were determined to contribute to improved oxidation resistance through interaction with dopant elements. This interaction is interpreted to be due to the contribution of Al and Si in formation of stable surface oxides.
Dopants. Dopants are elements found to have a major effect on the protective nature of the host oxide. Typically, they are found in groups IA, IIA and IIIB of the Periodic Table of Elements and include, without limitation, those described herein, as well as mixtures of these materials. Their function in improving oxidation resistance is judged to be due to their effect on predominantly anion and to a lesser extent cation transport through the surface oxide film.